Two kinds of studies will be undertaken: 1. Electron microscopic investigations of the sequence of bases in DNA. In the past we have learned to make nucleic acid preparation with bases specifically marked with small clusters of heavy atoms and the macromolecules linearly extended. Micrographs of such molecules will be collected. From them the positions of the different bases and hence the sequence will be deduced. 2. The energy analysing high resolution (Crewe type) electron microscope will be used to study the structure of various nucleoprotein preparations, most particularly eukaryotic chromosomes. Before this can be undertaken rationally, we must make empirical observations on the energy loss spectra of proteins, nucleic acids and their constituents. This will be done.